1. Field of the Invention
The present invention relates generally to electrotherapy devices and, more particularly, to measuring patient impedance in an electrotherapy device.
2. Related Art
Electrotherapy devices are used to provide electrical shocks to treat patients for a variety of heart arrhythmias. For example, external defibrillators typically provide high-energy shocks to a patient, usually through a pair of electrodes attached to the patient""s torso. External defibrillators are used to convert ventricular fibrillation or shockable tachycardia to a normal sinus rhythm. Similarly, external and internal cardioverters can be used to provide shocks to convert atrial fibrillation to a more normal heart rhythm.
Conventional external defibrillators have been used primarily in hospitals and other medical care facilities. While these external defibrillators have been known for years, they have generally been large and expensive making them unsuitable for use outside of a medical care facility. More recently, portable external defibrillators for use by first responders have been developed. Portable defibrillators allow medical care to be provided to a patient at the patient""s location considerably earlier than preceding defibrillators, increasing the likelihood of survival.
With recent advances in technology, portable defibrillators have become more automated, allowing even a minimally trained operator to use such devices to aid a heart attack victim in the critical first few minutes subsequent to the onset of sudden cardiac arrest. Such portable defibrillators, referred to as automatic or semi-automatic external defibrillators (generally, AEDs), may be stored in an accessible location at a business, home, aircraft or the like.
Generally, manual external defibrillators are configured by an operator for the particular patient and patient condition. In contrast, such determinations are made by the AED for the patient. One of the configuration parameters that needs to be determined is before administering a defibrillating pulse is the energy to be delivered by that pulse. Most AEDs use a fixed energy level. Many of today""s AEDs make some level of adjustment of the defibrillation waveform to compensate for different levels of patient impedance. Typically, conventional approaches for measuring patient impedance in electrotherapy devices involve driving the electrodes with a high impedance current source at a frequency greater than 500 Hz and measuring the voltage across the electrodes at the frequency of the current source. From this, the impedance of the current path that includes the two electrodes is determined.
Such an impedance measurement may provide useful information for certain purposes such as to estimate the impedance of the entire defibrillator path, heart rate, respiratory rate and other physiological parameters. However, such an approach fails to provide the information necessary to make an accurate estimation of patient size. For example, it is not uncommon for the impedance value measured using such traditional techniques to be approximately the same for a large adult male and a pediatric patient. Furthermore, the impedance measured in a two-electrode system will increase due to poor electrode contact interfering with the accurate determination of actual body impedance. It follows, then, that such measured values are insufficient to differentiate between patients of different body mass and to determine the optimal defibrillating current to be applied to a given patient.
The present invention is directed to an electrotherapy device with an improved apparatus and methodology for accurately measuring patient impedance. The invention implements a resistive network model of the patient""s body that includes one or more equations including resistive elements each representing the impedance components of the current paths through the patient. The present invention utilizes at least three electrodes placed at predetermined relative locations on the patient""s body, and measures the voltage across different electrode pairs while an applying an alternating current through certain electrodes. The applied current and measured voltages are used to solve the patient model equations for the individual resistive elements. Thus, each individual impedance component is separately and accurately determined.
There are numerous benefits provided by the determination of patient impedance separate from the other impedance values in the current flow path(s). In contrast to conventional approaches in which a single impedance value is determined for all current flow paths through the patient, the patient impedance generated in accordance with the present invention is not lumped or combined with other impedance values such as electrode-to-skin impedance. As a result, the patient impedance determined by the present invention is more accurate and, therefore, can be used to accurately determine patient size and the optimal energy to be delivered with an applied pulse. Similarly, the impedance values determined in accordance with the present invention can also be utilized to increase the accuracy of other determinations such as respiratory rate, cardiac output, proper electrode placement, effects of CPR and the like.
A number of aspects of the invention are summarized below, along with different embodiments that may be implemented for each of the summarized aspects. It should be understood that the summarized embodiments are not necessarily inclusive or exclusive of each other and may be combined in any manner in connection with the same or different aspects that is non-conflicting and otherwise possible. These disclosed aspects of the invention, which are directed primarily to systems, methods, data and techniques related to measuring bioelectrical impedance, are exemplary aspects only and are also to be considered non-limiting.
In one aspect of the invention, a multivariate impedance measurement module for use in an electrotherapy device such as a cardioverter, a defibrillator and a pacemaker is disclosed. The multivariate impedance measurement module implements a resistive network model of a patient""s body that is defined by voltage/current equations having terms representing an impedance of current paths between electrodes through the patient. In one embodiment, the measurement module utilizes at least three electrodes placed at predetermined relative locations on the patient""s body. The module successively measures a voltage across different pairs of the electrodes while applying an alternating current through a selected pair of electrodes.
The resistive network patient model includes resistive elements each representing an impedance of a current path that connects nodes of the network model. The nodes include the electrodes.
The resistance elements can include a plurality of resistive elements each representing an electrode/skin impedance between each of the at least three electrodes and a corresponding location at which current delivered by each the electrode is delivered into the patient""s body. The resistance elements can also or alternatively include an impedance of each current path from a location at which current is delivered into the patient""s body and a geometric center of the patient model and an impedance of a current path from a first location at which current is delivered into the patient""s body to a second location at which current exists the patient""s body.
In another aspect of the invention, an electrotherapy device for applying a therapeutic shock to a patient such as a cardioverter, a defibrillator and a pacemaker is disclosed. The module includes at least three electrodes for placement in a predetermined relative position on the patient""s body; and a multivariate impedance measurement module that applies an alternating current through one or more electrode pairs while concurrently measuring a voltage across successive electrode pairs.
The electrodes are placed at predetermined relative locations on the patient""s body such that a geometric center of the electrodes is approximately located at the patient""s heart. The module implements a resistive network model of the patient that includes resistive elements each representing an impedance of a current path between nodes of the network model. The patient model is defined by a plurality of voltage and/or current equations each including terms representing the impedance of the current paths. The module utilizes the applied current and measured voltages to solve the plurality of resistive network patient model equations for the unknown impedance terms.
The resistive elements can include, for example, a plurality of resistive elements each representing an electrode/skin impedance between each electrode and a corresponding location at which current delivered by that electrode is delivered into the patient""s body; an impedance of each current path from a location at which current is delivered into the patient""s body and the geometric center of the patient model or an impedance of a current path from a first location at which current is delivered into the patient""s body to a second location at which current exists the patient""s body.
In a still further aspect of the invention, a method for measuring patient impedance is disclosed. The method includes (1) applying at least three electrodes to a patient""s body at predetermined relative locations such that a geometric center of the electrodes is located approximately at the location of the natural heart; and (2) applying an alternating current to a first pair of electrodes. In addition, the method includes (3) successively measuring voltage across a plurality of pairs of electrodes during the application of the alternating current; and (4) solving a resistive network model defined by a voltage and current equations each expressed in terms of unknown resistance values each representing an impedance component along a current path through the patient from one electrode to another electrode of the first electrode pair.